1. Field of the Invention
This invention relates to a semiconductor memory device, especially relates to a memory device, in which write sequence is controlled with a verify-read operation.
2. Description of Related Art
A flash memory, which is one of electrically rewritable and non-volatile semiconductor memory devices (EEPROMs), stores data in a non-volatile manner in accordance with charge storing states of a charge storing layer (e.g., floating gate) of a memory cell. For example, the device stores binary data in such a manner that a low threshold voltage state (usually, a negative threshold voltage state) of a memory cell is defined as data “1”, which is obtained by releasing electrons of the floating gate, while a high threshold voltage state (usually, a positive threshold voltage state) is defined as data “0”, which is obtained by injecting electrons into the floating gate.
A NAND-type flash memory is known as one of flash memories. The NAND-type flash memory may be formed to have a large memory capacity because plural memory cells are connected in series in a manner that adjacent two memory cells share a source/drain diffusion layer.
To further increase the capacity of the flash memory device, a multi-value storage scheme, in which a memory cell stores multi bits, is used. For example, in a four-value storage flash memory, in which two bits are stored in a memory cell, one of “11”, “10”, “01” and “00” data, which are defined as being arranged in order of threshold voltage height, will be written.
Data write of the flash memory is performed with applying a write voltage, which is necessary for causing electron injection into a floating gate of a selected memory cell. To set the written threshold voltages of memory cells within a certain range, it is required to repeat a write voltage application and a write-verify for verifying the written state. Further, with increasing the write voltage by a small voltage step for each write cycle, it becomes possible to precisely control the written threshold voltage.
It has been achieved a substantially high-rate data write in the NAND-type flash memory by use of a scheme that data write is done by a page. At a write time of the NAND-type flash memory, a write voltage Vpgm is applied to a selected word line corresponding to a selected page in a selected block; and a write pass voltage Vpass to at least unselected (i.e., non-selected) word line(s) located on the bit line side, which is possible to turn on memory cells without regard to cell data, but insufficient for causing electron injection into unselected memory cells. At a write-verify time, a verify-read voltage Vvp is applied to the selected word line; and a read pass voltage Vread to unselected word lines, which turns on the unselected memory cells without regard to cell data.
As described above, it is required of a flash memory to execute plural write cycles in order to achieve certain data threshold distributions, and this fact puts a basic restriction on further improving the high-rate write performance of the flash memory.
There has been provided one approach, for further improving the high-rate write performance of the flash memory, as to shorten the write-verify time in consideration of the pass voltage applied to unselected word lines in the write-verify time (e.g., refer to Published and Unexamined Japanese Patent Application No. 2002-133888).
The write-verify read of the flash memory is, as similar to the ordinary read, done as to detect a discharge state of a bit line, which has been precharged to a certain voltage, thereby judging data. Bit line capacitance becomes larger as the memory cell array is more miniaturized and capacity thereof is more increased. This results in that it takes a long time for charging up the bit line, and it is difficult to shorten the write-verify time. Power consumption of the flash memory is usually defined in the specifications, and this also prevents the write-verify from being shortened.